Horns that include a membrane to produce sound through vibration are generally known in the art. For example, U.S. Pat. No. 870,874 to Astrom, incorporated herein by reference in its entirety, discloses a horn including an outer vessel and an inner vessel concentrically disposed therein. A gap exists between the vessels, with the outer vessel connected to the inner vessel at the base of the outer vessel. A pipe having a channel in communication with the gap extends from the outer vessel. In addition, a countersunk cap holds a diaphragm tautly against the upper edges of the inner and outer vessels. In use, air is forced through the pipe, enters the gap and travels toward the diaphragm. The pressure caused by the airflow forces the diaphragm away from the edge of the inner vessel, which, in turn, allows the air to enter the inner vessel passageway. Once the air enters the passageway, it expands, increasing in velocity. This creates a low pressure region that pulls the diaphragm back toward the edge of the inner vessel. The diaphragm remains positioned against the edge of the inner vessel until the pressure from the airflow is again sufficient to force the diaphragm away from the edge. The process repeats in a cyclic manner for as long as the forced air is applied and drawn over the diaphragm, causing it to vibrate at audible frequencies, and produce sound.
U.S. Pat. No. 5,460,116 to Gyorgy, incorporated herein by reference in its entirety, discloses a horn including a sound tube coaxially surrounded by a pressure tube such that an annular gap exists between the tubes, the gap having a minimum clearance of 0.2 mm. A closing collar holds the tubes together at one end, while a membrane is stretched over the opposite ends. The membrane is held in place by a retaining ring that is force-fit into a step located on the exterior of the pressure tube. In use, air is forced through a lateral opening in the pressure tube. The air causes the membrane to vibrate, which, in turn, generates sound.
Similarly, U.S. Pat. No. 5,662,064, also to Gyorgy, incorporated herein by reference in its entirety, discloses a horn including a sound tube coaxially surrounded by a pressure tube such that a gap exists between the tubes. The upper end of the sound tube is set back from the upper end of the pressure tube. A membrane is stretched over the upper ends of the tubes. A ring secures the membrane to the pressure tube, disposing the membrane against the edge of the sound tube. In use, air is forced through a lateral opening in the pressure tube, causing the membrane to vibrate, which, in turn, generates sound.
While each of the horns described above provides certain efficiencies and advantages, there still exists a need to provide a horn that is small and lightweight, but is able to produce a sound having variable frequencies. The horns of Gyorgy, for example, lack an end cap. As a result, the sound produced is weaker, becoming lost in the noise pollution of the surrounding environment, such as that existing at an athletic event. In addition, none of the Gyorgy or Astrom horns includes an adjustable end cap configured to alter the nature of the sound produced by the horn (e.g., its frequency, tone, pitch, etc). Consequently, there exists a need to provide a portable, lightweight acoustic device capable of producing high volume sound, and which is further capable of producing sound having varying frequency.
This invention is directed generally to a handheld acoustic device including a membrane and a repositionable end cap disposed over the membrane. More specifically, this invention is directed toward an acoustic device including an end cap whose cover portion can be positioned at varying axial displacement relative to a membrane to alter the frequency of the sound produced by the device.